1. Field of the Invention
The present invention relates to a method of manufacturing a multi-layer resin material such as a multi-layer resin sheet or film.
2. Description of the Prior Art
A multi-layer resin sheet or film has heretofore been produced by kneading and melting a plurality of different kinds of resins using a plurality of extrusion machines and then conjointly extruding the kneaded resins together from a die in the shape of a multi-layer structure. One example of known multi-layer resin materials is composed of an inner layer made of a gas barrier resin, for example, a resin having low oxygen permeability such as ethylene-vinyl alcohol copolymer, and an outer layer made of a polyolefin resin which may be bonded to the inner layer by means of an adhesive resin, if necessary.
In the manufacture of a multi-layer resin material, the thickness of each of the resin layers must be uniform over the entire width of the material, and it is also necessary to trim both lateral edges of the multi-layer resin material in order to obtain a product having a uniform thickness and width. When a multi-layer resin material is stretched using a tenter as a secondary stretching process, the edge portions of this material which are grasped by the chucks of the tenter are rejected as scrap. If the product is made of a single kind of resin, this scrap can be re-used by processing it through a scrap grinder. In the case of a multi-layer resin material, however, the scrap is a laminate of different kinds of resins which are united together over their entire widths, and it is, therefore, difficult to re-use that scrap. Multi-layer resin materials which are composed of resins that differ in heat resistance are particularly difficult to re-use and, therefore, involve the problem of wasteful use of resources and increases in costs.
In view of the above-described circumstances, there have already been proposed the following two methods as means for enabling a multi-layer material extruded from a die to have lateral edge portions which are virtually completely made of only an outer-layer resin: (1) the die is specially designed for the above-described purposes (disclosed in Japanese Patent Publication Nos. 5738/1974, 26216/1982, 3809/1983 and 1179/1984); and (2) an inner-layer resin is wrapped or encircled by an outerlayer resin within a feed block (disclosed in Japanese Patent Publication No. 26216/1982).
These proposed methods suffer, however, from the following disadvantages. With the method 1), the die structure is complicated, and it is not possible to satisfactorily cope with changes in the kinds of resins employed, the layer thickness ratio, operating conditions and so forth. With the method (2), it is difficult to make uniform the thickness distribution of each of the resin layers over the entire width and it is also difficult to satisfactorily form a single-resin layer at each lateral edge.